The present invention relates to a positive-temperature-coefficient (PTC) thermistor heating device which has a high and stable thermal output and a process for fabricating the same.
Use of positive-temperature-coefficient (abbreviated as "PTC" in this specification) thermistor elements as heat sources are advantageous in that because of their "self-temperature-control action", overheating can be avoided and temperature variations are minimum. The thermal output (W) of a PTC thermistor is given by EQU W=C(T-T.sub.a)
where C=a coefficient of heat or thermal radiation,
T=a surface temperature of the thermistor, and
T.sub.a =an ambient temperature.
The surface temperature T of a PTC thermistor element becomes almost constant at or in the proximity of a Curie point of the thermistor element, so that in order to increase the thermal output W, the coefficient of heat or thermal radiation C must be increased. To this end, it has been a universal practice to join to the electrodes on a PTC thermistor element heat radiating means which are made of a metal or a metal alloy and which serve to increase the coefficient of thermal radiation C.
However, the prior art PTC thermistor heating device with metal heat radiating means have problems to be described below.
(1) In order to join metal heat radiating means to the electrodes of a PTC thermistor element to obtain thereby the highest thermal output, the surfaces of contact between the heat radiating means and the electrodes of the thermistor element must be ground and/or polished flat so that they are in very intimate contact with each other. As a result, the fabrication steps are increased in number with a resultant increase in fabrication costs.
(2) In some types of PTC thermistor heating devices, a bias spring is used to press the metal heat radiating means against the electrodes on the PTC thermistor element. However, the bias spring is easily susceptible to thermal fatigue, so that the biasing force applied to the metal heat radiating means is reduced.
(3) In some types of PTC thermistor heating devices, a PTC thermistor element, metal heat radiating means and a bias spring are mounted in an insulation frame. The frame is subjected to thermal creep due to temperature variations so that the pressure of contact between the metal heat radiating means and the PTC thermistor element varies and consequently the internal electrical resistance and hence the thermal output of the heating element varies.
(4) In some types of PTC thermistor heating devices, a bond between a PTC thermistor element and metal heat radiating means is obtained with an adhesive which is electrically conductive. However, such an adhesive as described above is very expensive. In addition, the adhesive bond is easily susceptible to breakage due to mechanical impact or vibration. Furthermore, if the adhesive drips or is squeezed out to bridge across electrically isolated parts, short-circuits result.